FIG. 6 illustrates an example of a conventional pulse signal measuring instrument ordinarily used to measure the pulse width and the signal period of an input signal, and the time interval between two input signals. FIG. 7A shows the timing diagram of each signal obtained during calibration of the conventional time interval measuring instrument for a signal period defined by positive edges. FIG. 7B shows the timing diagram of each signal obtained during calibration of the conventional time interval measuring instrument for a signal period defined by negative edges.
The conventional pulse measuring instrument generally is comprised of input terminals 11 and 12, a change-over switch 13, comparators 16 and 17, a gate circuit 22, a measuring part 25, a calculation part (CPU) 52, a display 30 and D/A converters 31 and 32. The measuring part 25 is made up of input terminals 23 and 24, D-type flip flops 26 and 27 and a time difference measuring circuit 28. The input terminal 11 where the pulse signal is supplied is connected to a noninverting input of the comparator 16 through a terminal 14. The input terminal 12 is connected to a noninverting input of the comparator 17 through the change-over switch 13 and a terminal 15. Inverting inputs of the comparators 16 and 17 are connected to outputs of the D/A converters 31 and 32 respectively. The change-over switch 13 has contacts 13.sub.1, 13.sub.2 and 13.sub.3. Contact 13.sub.1 is connected to the input terminal 11 and contact 13.sub.2 is connected to input terminal 12, respectively. Contact 13.sub.3 is connected to the noninverting input of the comparator 17. The D/A converters 31 and 32 respectively provide intermediate level voltages V.sub.TA and V.sub.TB to inverting inputs of the comparators 16 and 17. Each of the comparators has a noninverting output and an inverting output. Delay elements 18, 19, 50 and 51 are connected to the outputs of the comparators 16 and 17. The gate circuit 22 has a first gate circuit and a second gate circuit. The first gate circuit interchangeably provides either the noninverting output or the inverting output from the comparator 16 to the measuring part 25 in response to a control signal S.sub.31. The second gate circuit interchangeably provides either the noninverting output or the inverting output from the comparator 17 to the measuring part 25 in response to a control signal S.sub.32. Examples of the circuit configuration for the time difference measuring circuit 28 in the measuring part 25 is described in Japanese Patent Publication No. 63-3272 and Japanese Patent Laying-Open Publication No. 62-294993.
In the conventional pulse measuring instrument of FIG. 6, the change-over switch 13 and the control signals S.sub.31 and S.sub.32 are set as shown in Table 1, depending upon the measuring items and the edges of the input signals.
TABLE 1 ______________________________________ Signal Signal Measurement Switch 13 S.sub.31 S.sub.32 ______________________________________ Positive edge signal period Input terminal 11 H H Negative edge signal period Input terminal 11 L L Positive pulse width Input terminal 11 H L Negative pulse width Input terminal 11 L H Positive edge time interval Input terminal 12 H H Negative edge time interval Input terminal 12 L L ______________________________________
In FIG. 6, there are three signal paths from the input terminals 11 and 12 to the terminals 14 and 15. The first path is a route from the input terminal 11 to the terminal 14. The second path is a route from the input terminal 11, to the change-over switch 13, to the terminal 15. The third path is a route from the input terminal 12, to the change-over switch 13, to the terminal 15. Signal propagation times in the above three paths are substantially identical. Usually, there is no difficulty in making the signal propagation times equal, since these signal paths are formed by small passive elements. However, there are significant amount of time differences between signal paths which include the comparators 16, 17 and the gate circuit 22. The delay elements 18, 19, 50, and 51, shown between the comparators 16 and 17 and the gate circuit 22, represent delay times, i.e., the signal propagation times in corresponding signal paths. Further, these delay elements indicate the signal propagation of times .tau..sub.A.sup.+, .tau..sub.A.sup.-, .tau..sub.B.sup.+, .tau..sub.B.sup.- of the signal paths. The delay elements 18 and 19 do not necessarily need to be independent or discrete elements but, rather, they need to show the total amount of delay time in the corresponding signal paths. The delay elements 50 and 51 are formed of, for example, delay lines, typically coaxial cables.
In the example of FIG. 6, the signal propagation time in a signal path extending from the terminal 14, to the non-inverting output of the comparator 16, to the gate circuit 22, and, finally, to the input terminal 23, is designated as .tau..sub.A.sup.+. In contrast, the signal propagation time in a signal path extending from the terminal 14, to the inverting output of the comparator 16, to the gate circuit 22, and, finally, to the input terminal 23, is designated as .tau..sub.A.sup.-. Further, the signal propagation time in a signal path extending from the terminal 15, to the non-inverting output of the comparator 17, to the gate circuit 22, and ending at the input terminal 24, is designated as .tau..sub.B.sup.+. However, the signal propagation time in a signal path extending from the terminal 15, to the inverting output of the comparator 17, to the gate circuit 22, terminating at the input terminal 24, is designated as .tau..sub.B.sup.-. The delay times in the delay elements 50 and 51 are variable by adjusting, for example, the cable length (not shown). The measuring part 25 is calibrated so that it can accurately measure the time difference between the positive edges of signal S.sub.21 and S.sub.22, which are input to input terminals 23 and 24, respectively.
When the change-over switch 13 is switched to the contact 13.sub.1, a pulse signal S.sub.11 (which is to be measured) from input terminal 11 is supplied to the comparators 16 and 17. This is a typical situation for measuring the signal period and the pulse width of the input signal S.sub.11. When the change-over switch 13 is switched to the contact 13.sub.2, two pulse signals are input to the input terminals 11 and 12, respectively. Thus, one of the pulse signals is supplied to the comparator 16 and the other pulse signal is supplied to the comparator 17. This is a situation for measuring the time interval between the two signals S.sub.11 and S.sub.12. The comparators 16 and 17 output the pulse signals by formatting the input signals into the square waves having sharp rising and falling edges. The output signals from each of the comparators 16 and 17 are in the inverting form and in the noninverting form. The output signals from the comparators 16 and 17 are sent to the gate circuit 22 in the form of signals S.sub.21 and S.sub.22. This selection is accomplished by the control signals S.sub.31 and S.sub.32 from the CPU 52. The signals S.sub.21 and S.sub.22 are input to the measuring part 25 through input terminals 23 and 24. The time difference measuring circuit 28 measures the time difference between the rising edges (positive edges) or the falling edges (negative edges) of the signals S.sub.21 and S.sub.22, which is either the signal period, the pulse width, or the time interval of the pulse signals. The CPU 52 instructs the display 30 to display the measured value.
In referring to FIGS. 6 and 7, the following is a procedure for calibrating a conventional pulse signal measuring instrument. First, the description is given regarding the calibration procedure for the measurement of the signal period between positive edges. In FIG. 7, it is assumed that the accurate values of the signal periods P.sup.+ (FIG. 7A) and P.sup.- (FIG. 7B) of the calibration signals S.sub.11 and S.sub.12 are known. Further, Pm.sup.+ represents the measured value of the positive edge signal period displayed on the display 30. Based on the waveforms in FIG. 7A, the following relationship is established between the measured value Pm.sup.+ and the accurate (true) signal period P.sup.+ of the calibration signal: EQU P.sup.+ +.tau..sub.B.sup.+ =.tau..sub.A.sup.+ +Pm.sup.+ EQU Pm.sup.+ =P.sup.+ +.tau..sub.B.sup.+ -.tau..sub.A.sup.+ ( 1)
From Equation (1), by adjusting the length of the coaxial cable (the delay element 50) to make the signal propagation time .tau..sub.B.sup.+ equal to the signal propagation time .tau..sub.A.sup.+, the measured value Pm.sup.+ can be calibrated to the accurate signal period P.sup.+. Thus, after this calibration, accurate measurement of signal period between positive edges for an input signal can be made using the pulse measuring instrument.
The calibration of signal period measurements taken between negative edges is performed in a manner similar to the above-described procedure. With reference to FIG. 7B, the signal period Pm.sup.- represents the measured value of the negative edge signal period displayed on the display 30. Based on the waveforms in FIG. 7B, the following relationship is established between the measured value Pm.sup.- and the accurate (true) signal period P.sup.- : EQU P.sup.- +.tau..sub.B.sup.- =.tau..sub.A.sup.- +Pm.sup.- EQU Pm.sup.- =P.sup.- +.tau..sub.B.sup.- -.tau..sub.A.sup.- ( 2)
From Equation (2), by adjusting the length of the coaxial cable (the delay element 51), thereby making the signal propagation time .tau..sub.B.sup.- equal to the signal propagation time .tau..sub.A.sup.-, the measured value Pm.sup.+ can be calibrated to the actual signal period P.sup.-. Thus, after this calibration, accurate measurement of signal period between positive edges for an input signal can be made using the pulse measuring instrument.
The calibration of pulse width measurements is performed in a manner similar to the calibration of the signal period measurements, as described above. For example, FIG. 8 shows the timing relationship between the input calibration pulse and the internal pulse signals which are affected by the signal propagation times. The timing chart of FIG. 8 is directed to the calibration of positive pulse width measurements, wherein the relationship between the measured pulse width Wm.sup.+- and the accurate pulse width Wx.sup.+- is established as follows: EQU Wm.sup.+- =Wx.sup.+- +.tau..sub.B.sup.- -.tau..sub.A.sup.+ ( 3)
Thus, in order to remove (.tau..sub.B.sup.- -.tau..sub.A.sup.+) from Equation (3), the length of the coaxial cable (the delay element 51) is adjusted so that the propagation time .tau..sub.B.sup.- is equal to the propagation time .tau..sub.A.sup.+. Similarly, in the calibration of negative pulse width Wm.sup.-+ measurements, the signal propagation time .tau..sub.B.sup.+ is adjusted to be equal to the signal propagation time .tau..sub.A.sup.-.
In the above mentioned conventional pulse measuring instrument, the calibration is performed by adjusting the length of the coaxial cable (the delay elements 50 and 51) with reference to the propagation time differences between the two pairs of signal paths, i.e., the signal paths for the positive edges and the signal paths for the negative edges. However, this procedure is cumbersome and time consuming since this adjustment of propagation time is undertaken manually. In addition, even though the signal propagation times are accurately adjusted to .tau..sub.B.sup.+ =.tau..sub.A.sup.+ and .tau..sub.B.sup.- =.tau..sub.A.sup.-, the calibration will be affected by temperature changes and/or time intervals between calibrations. As a result, the signal propagation times may be affected by temperature changes or time intervals between calibrations. Therefore, it is difficult to maintain an accurate measurement for a reasonably long period of time. Furthermore, calibration of the pulse width measurement, as described above, requires a pulse generator having an accurate pulse width. However, although a pulse generator can usually generate a signal having an accurate signal period or frequency, it is difficult to generate a pulse signal having an accurate pulse width. Thus, such a pulse generator is either unavailable or extremely expensive.